A Montana prison is being sued for refusing to pass along Spanish-language letters, and thereby violating a prisoner’s rights.

The American Civil Liberties Union is claiming that the free speech rights of William Diaz-Wassmer, 26, were violated when prison employees refused to pass along letters from the prisoner because they were written in Spanish.

The prison’s policy is that all mail must be screened to assure they contain no security threats. If the mail is written in code or in a language the officials do not understand, it is returned to sender.

In Diaz-Wassmer’s case, his mail was often written in Spanish, and the prison workers could not read Spanish the letters were returned.

In the state’s court filings, they stated, “Inmate correspondence can be used to plot threats to the facility, its staff, other inmates and the public at large. Adding,“Inmate correspondence written in code or in a language which prison officials cannot understand can be used to facilitate the commission of blackmail, extortion, escape plans, trafficking in contraband and prison assaults and disturbances.”

The ACLU claims the prison’s “English-only” policy violates First Amendment right to free speech and his Fourteenth Amendment Right to equal protection.

However, the prison denies such a policy is in place and says the letters were returned because they lack the funds to hire a translator.

William Diaz-Wassmer has been serving a life term at Montana State Prison since 2007. For the first two years of his incarceration, Diaz-Wassmer was able to correspond with his parents, family and friends in their native language of Spanish. But starting in May 2010 he was prohibited from receiving mail in any language but English.

Diaz-Wassmer, who is original from Guatemala, is said to be fluent in English, and his family members understand some English.

ACLU of Montana staff attorney Jennifer Giuttari told Montana’s KRTV3, “Courts have recognized that written correspondence can be very beneficial to an inmate’s morale or to helping them adjust to life in an institution. We believe that inmates still retain their constitutional rights despite that they are incarcerated.”